Recieving a nullpointerexception while following the Unity Instantiation tutorial

I am coding this in C#, and I ran into this issue while coding a simple script to shoot a ball from inside of my character. I have figured out it is coming from the AddForce function after casting as a Rigidbody, but can’t figure out which value is null.

    using UnityEngine;
using System.Collections;

public class Shoot : MonoBehaviour {

	public Rigidbody bulletPrefab;
	public Transform playerCenter;

	void Update() {
		if(Input.GetButtonDown("Fire1")) {
			Rigidbody bulletSpawn = Instantiate(bulletPrefab,							//The prefab spawned
			                          playerCenter.position,				//Gets the position the prefab will go
			                          playerCenter.rotation) as Rigidbody;	//Gets the rotation the prefab will go

			bulletSpawn.AddForce(playerCenter.forward * 5000);



it could be bulletPrefab or playerCenter. Make sure these properties are filled in in your inspector.